Nick, If you look at Liz's post above you'll see that this very popular minstrel song of 1850s has taken the form, tune, and some of the nonsense chorus from the earlier stream of British songs and some of this has become attached to the 'Frog went a courtin' text. It's easy to see how this could happen. Any song written in couplets with lines of that length could be adapted to any number of tune/chorus/formats available in oral tradition.